gskit: remove

We remove support for building curl with gskit.

 - This is a niche TLS library, only running on some IBM systems
 - no regular curl contributors use this backend
 - no CI builds use or verify this backend
 - gskit, or the curl adaption for it, lacks many modern TLS features
   making it an inferior solution
 - build breakages in this code take weeks or more to get detected
 - fixing gskit code is mostly done "flying blind"

This removal has been advertized in DEPRECATED in Jan 2, 2023 and it has
been mentioned on the curl-library mailing list.

It could be brought back, this is not a ban. Given proper effort and
will, gskit support is welcome back into the curl TLS backend family.

Closes #11460

docs/libcurl/curl_global_sslset.3

@@ -38,7 +38,7 @@ typedef enum {
   CURLSSLBACKEND_OPENSSL = 1, /* or one of its forks */
   CURLSSLBACKEND_GNUTLS = 2,
   CURLSSLBACKEND_NSS = 3,
-  CURLSSLBACKEND_GSKIT = 5,
+  CURLSSLBACKEND_GSKIT = 5, /* deprecated */
   CURLSSLBACKEND_POLARSSL = 6, /* deprecated */
   CURLSSLBACKEND_WOLFSSL = 7,
   CURLSSLBACKEND_SCHANNEL = 8,

docs/libcurl/libcurl-env.3

@@ -50,7 +50,7 @@ specific backend at first use. If no selection is done by the program using
 libcurl, this variable's selection will be used. Setting a name that is not a
 built-in alternative will make libcurl stay with the default.
 
-SSL backend names (case-insensitive): BearSSL, GnuTLS, gskit, mbedTLS,
+SSL backend names (case-insensitive): BearSSL, GnuTLS, mbedTLS,
 nss, OpenSSL, rustls, Schannel, Secure-Transport, wolfSSL
 .IP HOME
 When the netrc feature is used (\fICURLOPT_NETRC(3)\fP), this variable is

docs/libcurl/opts/CURLINFO_CERTINFO.3

@@ -75,7 +75,7 @@ if(curl) {
 }
 .fi
 .SH AVAILABILITY
-This option is only working in libcurl built with OpenSSL, Schannel, GSKit or
+This option is only working in libcurl built with OpenSSL, Schannel or
 Secure Transport support. Schannel support added in 7.50.0. Secure Transport
 support added in 7.79.0.
 

docs/libcurl/opts/CURLINFO_TLS_SESSION.3

@@ -63,8 +63,8 @@ if(curl) {
 }
 .fi
 .SH AVAILABILITY
-Added in 7.34.0. Deprecated since 7.48.0 and supported OpenSSL, GnuTLS,
-NSS and gskit only up until this version was released.
+Added in 7.34.0. Deprecated since 7.48.0 and supported OpenSSL, GnuTLS, and
+NSS only up until this version was released.
 .SH RETURN VALUE
 Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.
 .SH "SEE ALSO"

docs/libcurl/opts/CURLINFO_TLS_SSL_PTR.3

@@ -57,18 +57,15 @@ struct curl_tlssessioninfo {
 The \fIbackend\fP struct member is one of the defines in the CURLSSLBACKEND_*
 series: CURLSSLBACKEND_NONE (when built without TLS support),
 CURLSSLBACKEND_WOLFSSL, CURLSSLBACKEND_SECURETRANSPORT, CURLSSLBACKEND_GNUTLS,
-CURLSSLBACKEND_GSKIT, CURLSSLBACKEND_MBEDTLS, CURLSSLBACKEND_NSS,
-CURLSSLBACKEND_OPENSSL, CURLSSLBACKEND_SCHANNEL or
-CURLSSLBACKEND_MESALINK. (Note that the OpenSSL forks are all reported as just
-OpenSSL here.)
+CURLSSLBACKEND_MBEDTLS, CURLSSLBACKEND_NSS, CURLSSLBACKEND_OPENSSL,
+CURLSSLBACKEND_SCHANNEL or CURLSSLBACKEND_MESALINK. (Note that the OpenSSL
+forks are all reported as just OpenSSL here.)
 
 The \fIinternals\fP struct member will point to a TLS library specific pointer
 for the active ("in use") SSL connection, with the following underlying types:
 .RS
 .IP GnuTLS
 \fBgnutls_session_t\fP
-.IP gskit
-\fBgsk_handle\fP
 .IP NSS
 \fBPRFileDesc *\fP
 .IP OpenSSL

docs/libcurl/opts/CURLOPT_CERTINFO.3

@@ -74,7 +74,7 @@ if(curl) {
 }
 .fi
 .SH AVAILABILITY
-This option is supported by the OpenSSL, GnuTLS, Schannel, GSKit and Secure
+This option is supported by the OpenSSL, GnuTLS, Schannel and Secure
 Transport backends. Schannel support added in 7.50.0. Secure Transport support
 added in 7.79.0.
 .SH RETURN VALUE

docs/libcurl/opts/CURLOPT_PINNEDPUBLICKEY.3

@@ -102,8 +102,6 @@ PEM/DER support:
 
   7.39.0: OpenSSL, GnuTLS
 
-  7.39.0-7.48.0,7.58.1+: GSKit
-
   7.43.0: wolfSSL
 
   7.47.0: mbedTLS

docs/libcurl/opts/CURLOPT_PROXY_PINNEDPUBLICKEY.3

@@ -98,7 +98,7 @@ footer:
 .SH AVAILABILITY
 PEM/DER support:
 
-  7.52.0: GSKit, GnuTLS, OpenSSL, mbedTLS, wolfSSL
+  7.52.0: GnuTLS, OpenSSL, mbedTLS, wolfSSL
 
 sha256 support: