The current check caught stuff in the .ver file that was outdated, but
didn't quite get stuff that was supposed to be exported but wasn't,
in some cases.  Update the regex to ensure that symbols that are
not exported, but that start with "nm_" (and aren't GObject
boilerplate) get flagged during the check.

Update internal code to return to a previous pattern of treating symbols
that start with "_" internal, and expecting symbols that start with
"nm_" to be exported.

Also fixes cases where random glibc symbols would get flagged:

00040ff4 l     O *ABS*  00000000              .hidden _GLOBAL_OFFSET_TABLE_
00041380 l     O .data  00000000              .hidden __dso_handle
000406e8 l     O .dtors 00000000              .hidden __DTOR_END__
0000fbe4 l     F .text  00000000              .hidden __i686.get_pc_thunk.cx
0002f980 l     F .text  00000014              .hidden __stack_chk_fail_local
0000b1e7 l     F .text  00000000              .hidden __i686.get_pc_thunk.bx
00040e60 l     O *ABS*  00000000              .hidden _DYNAMIC