Fix a performance regression in query functions

This commit brings back the old code for W_CheckNumForNamePwad, using memcmp
instead of strncmp, which is readily inlined by modern C compilers and has
more acceptable performance in P_AddWadFile. Also removes some redundant
strlens, saving the result of one call instead.

This commit increases this branch's distance from SRB2 2.2, per Ashnal
and SteelT's note that I shouldn't worry too much about it, especially
when there's performance on the line.