Use /proc/self/cmdline on Solaris as well in shell_global_reexec_self()

Use /proc/self/cmdline on Solaris as well in shell_global_reexec_self()

Solaris 11.3.5 and later have a Linux-compatible implementation of
/proc/self/cmdline, so the code to use it can be enabled in gnome-shell
on Solaris - if used on an older OS, it simply fails to open the file
and returns without doing anything, just as the code did before enabling
this.

https://bugzilla.gnome.org/show_bug.cgi?id=776199

Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>