regcomp.c: Split #define into two

regcomp.c: Split #define into two

The syntethic start class regnode (SSC) and a bracketed character class
node share much of the same data structure, including a flags field,
and some of the same flag bits within it.  Currently, only
locale-related flags (under /l rules) are the same between the two
during construction of the SSC.  But a future commit will introduce
another common flag.  This commit creates an extra #define for use where
we want the common flags, while retaining the existing one for use where
we want the locale flags.  The new #define is just a copy of the
existing one, to be changed in the future commit.