[SV 59881] Handle vertical TAB consistently

[SV 59881] Handle vertical TAB consistently

While parsing makefiles get_next_mword() was treating VTAB as a word
character rather than a word separator.  However, when using
find_next_token(), for example in patsubst_expand_pat(), we treated
VTAB as a word separator causing multiple words to appear where we
didn't expect them.

* src/makeint.h (END_OF_TOKEN): Change from a loop to a boolean check.
* src/misc.c (end_of_token): Move the loop here.
* src/read.c (get_next_mword): Skip whitespace, not just blank, to
find the start of the word and use END_OF_TOKEN() to decide when the
current word is finished.