signatures: eliminate XSIGVAR, add KEY_sigvar

signatures: eliminate XSIGVAR, add KEY_sigvar

When I moved subroutine signature processing into perly.y with
v5.25.3-101-gd3d9da4, I added a new lexer PL_expect state, XSIGVAR.

This indicated, when about to parse a variable, that it was a signature
element rather than a my variable; in particular, it makes ($,...)
be toked as the lone sigil '$' rather than the punctuation variable '$,'.

However this is a bit heavy-handled; so instead this commit adds a
new allowed pseudo-keyword value to PL_in_my: as well as KEY_my, KEY_our and
KEY_state, it can now be KEY_sigvar. This is a less intrusive change
to the lexer.