Emulate the value of BmFLAGS() using SvTAIL().

Emulate the value of BmFLAGS() using SvTAIL().

Don't set BmFLAGS() in Perl_fbm_compile()

Originally fbm_compile() had an I32 flags argument, which seems to have been
part of case folding/locale improvements. bbce6d69784bf43b removed this.
SvTAIL() was only used in once place until c277df42229d99fe. 2779dcf1a3ceec16
added the U32 flags argument to fbm_compile(), not used until cf93c79d660ae36c.
That commit also added FBMcf_TAIL and FBMcf_TAIL{z,Z,DOLLAR} but didn't use the
last three. Additionally, it stored the BmFLAGS as part of the compiled table:

+       table[-1] = flags;              /* Not used yet */

f722798beaa43749 added FBMcf_TAIL_DOLLARM, renumbered FBMcf_TAIL{z,Z,DOLLAR},
but still didn't use anything other than FBMcf_TAIL.

The core, nothing on CPAN, and nothing visible to Google codesearch, has ever
used the 4 specialist flags. The only use is 0 or FBMcf_TAIL, which is in
lockstep with SvTAIL() of 0 or non-0.