Simplify Solaris port

Simplify Solaris port

This should avoid some configuration confusion as exemplified
by Jeffrey Walton’s recent bug report (Bug#42675).
* configure.ac (opsys): Simplify Solaris configuration by
not worrying about Solaris 9 and earlier, as they are no
longer supported by the Solaris developers.  This should
support Walton’s ‘./configure --build=x86_64-sun-solaris’.
Instead of bothering with ‘opsys=sol2-6’ and ‘opsys=sol2-10’,
just use ‘opsys=solaris’.  All uses changed.
(emacs_check_sunpro_c): Remove unused var.
* doc/misc/tramp.texi (Remote programs):
* etc/MACHINES, etc/PROBLEMS:
Modernize PATH for Oracle Developer Studio.
* etc/PROBLEMS: Move Solaris-related problems to legacy area,
except those that are still relevant.