CURLOPT_DEFAULT_PROTOCOL: added

- Add new option CURLOPT_DEFAULT_PROTOCOL to allow specifying a default
protocol for schemeless URLs.

- Add new tool option --proto-default to expose
CURLOPT_DEFAULT_PROTOCOL.

In the case of schemeless URLs libcurl will behave in this way:

When the option is used libcurl will use the supplied default.

When the option is not used, libcurl will follow its usual plan of
guessing from the hostname and falling back to 'http'.

docs/curl.1

@@ -1312,6 +1312,25 @@ This option can be used multiple times, in which case the effect is the same
 as concatenating the protocols into one instance of the option.
 
 (Added in 7.20.2)
+.IP "--proto-default <protocol>"
+Tells curl to use \fIprotocol\fP for any URL missing a scheme name.
+
+Example:
+
+.RS
+.IP "--proto-default https ftp.mozilla.org"
+https://ftp.mozilla.org
+.RE
+
+An unknown or unsupported protocol causes error
+\fICURLE_UNSUPPORTED_PROTOCOL\fP.
+
+This option does not change the default proxy protocol (http).
+
+Without this option curl would make a guess based on the host, see \fI--url\fP
+for details.
+
+(Added in 7.45.0)
 .IP "--proto-redir <protocols>"
 Tells curl to use the listed protocols on redirect. See --proto for how
 protocols are represented.
@@ -1773,6 +1792,12 @@ If this option is used several times, the last one will be used.
 Specify a URL to fetch. This option is mostly handy when you want to specify
 URL(s) in a config file.
 
+If the given URL is missing a scheme name (such as "http://" or "ftp://" etc)
+then curl will make a guess based on the host. If the outermost sub-domain name
+matches DICT, FTP, IMAP, LDAP, POP3 or SMTP then that protocol will be used,
+otherwise HTTP will be used. Since 7.45.0 guessing can be disabled by setting a
+default protocol, see \fI--proto-default\fP for details.
+
 This option may be used any number of times. To control where this URL is
 written, use the \fI-o, --output\fP or the \fI-O, --remote-name\fP options.
 .IP "-v, --verbose"