Add variable INSTALLFLAGS_EDIT used to remove arbitrary INSTALLFLAGS.

Add variable INSTALLFLAGS_EDIT used to remove arbitrary INSTALLFLAGS.

Specifically intended for removing -fschg ("INSTALLFLAGS_EDIT=:S/schg/uchg/")
this makes the NOFSCHG flag redundant.  NOFSCHG will still be honoured by
bsd.lib.mk but is valid for buildworld only.  NOFSCHG is still implemented in
the old way (ie. _not_ ".if NOFSCHG then { INSTALLFLAGS_EDIT+=:S/schg/,/ }"
to emphasize the fact that NOFSCHG is only supported in a limited
fashion and for buildworld.

The interface and implementation are such that future use of flags such
as sappnd can also be easily removed or altered (perhaps to uappnd).

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