strparse: switch to curl_off_t as base data type

- add hex and octal parsers to the Curl_str_* family
- make curlx_strtoofft use these parsers
- remove all use of strtol() and strtoul() in library code
- generally use Curl_str_* more than strtoofft, for stricter parsing
- supports 64-bit universally, instead of 'long' which differs in size
  between platforms

Extended the unit test 1664 to verify hex and octal parsing.

Closes #16336

tests/server/Makefile.inc

@@ -29,6 +29,8 @@ CURLX_SRCS = \
  ../../lib/mprintf.c \
  ../../lib/nonblock.c \
  ../../lib/strtoofft.c \
+ ../../lib/strparse.c \
+ ../../lib/strequal.c \
  ../../lib/warnless.c \
  ../../lib/timediff.c \
  ../../lib/dynbuf.c \
@@ -41,10 +43,12 @@ CURLX_HDRS = \
  ../../lib/curlx.h \
  ../../lib/nonblock.h \
  ../../lib/strtoofft.h \
+ ../../lib/strcase.h \
  ../../lib/warnless.h \
  ../../lib/timediff.h \
  ../../lib/curl_ctype.h \
  ../../lib/dynbuf.h \
+ ../../lib/strcase.h \
  ../../lib/strdup.h \
  ../../lib/curl_get_line.h \
  ../../lib/curl_multibyte.h