Refactor: libcrmcommon: New pcmk__md5sum()

To replace crm_md5sum().

This returns (gchar *) to avoid a completely pointless string copy.
At time of writing, later commits incrementally update the
non-deprecated callers to use (gchar *) as well without the need to make
a copy.

However, I'm undecided about whether to stick with that approach, or
just to keep calling pcmkstr_copy() in pcmkmd5sum(). It's a useless
copy, BUT it lets the rest of the call chain use (char *) without
worrying about g_free() and such.

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>