vstring: Make vStringPut*() usage more forgiving

vStringPut() and friends take an `unsigned char` as an `int`, similar
to ctype functions, in order to be easy to use with `fgetc()`-style
functions.

However, this API is also used on regular C strings, which require a
cast if the value is larger than 0x7F (127) on systems where `char` is
a signed type.

In order to make the API easier to use, as it's easy to forget the cast
when working with `char`, introduce wrapper macros that add the cast
when called with `char`.  The cast is conditional so the underlying
implementation can still verify the value is in the valid range when
called with an `int` (to catch erroneous calls with EOF or other values
that do not fit a `char`).

Note that this will still not work properly if the caller has an
incorrect cast on its side, like e.g. `vStringPut(s, (int) c)` where
`c` is a `char`, as there's only so many magic tricks up our sleeves.
These calls should be updated to either be `vStringPut(s, c)` with the
added macros, or `vStringPut(s, (unsigned char) c)` if one wants to be
explicit -- yet no need to go all the trouble to make them
`vStringPut(s, (int) (unsigned char) c)`, as the `unsigned char` to
`int` conversion is implicit and safe.

Based off a suggestion from Masatake YAMATO <yamato@redhat.com>

main/vstring.h

@@ -102,7 +102,7 @@ CTAGS_INLINE void vStringPutNewlinAgainUnsafe (vString *const string)
 	string->buffer [string->length++] = '\n';
 }
 
-CTAGS_INLINE void vStringPut (vString *const string, const int c)
+CTAGS_INLINE void vStringPutImpl (vString *const string, const int c)
 {
 	/* verify the given character is an unsigned char value */
 	Assert (c >= 0 && c <= 0xff);
@@ -115,8 +115,12 @@ CTAGS_INLINE void vStringPut (vString *const string, const int c)
 		string->buffer [++string->length] = '\0';
 }
 
-CTAGS_INLINE bool vStringPutWithLimit (vString *const string, const int c,
-									   unsigned int maxlen)
+#define vStringPut(s, c) (sizeof(c) == sizeof(char) \
+						  ? vStringPutImpl((s), (unsigned char) (c)) \
+						  : vStringPutImpl((s), (c)))
+
+CTAGS_INLINE bool vStringPutWithLimitImpl (vString *const string, const int c,
+										   unsigned int maxlen)
 {
 	if (vStringLength (string) < maxlen || maxlen == 0)
 	{
@@ -126,4 +130,9 @@ CTAGS_INLINE bool vStringPutWithLimit (vString *const string, const int c,
 	return false;
 }
 
+#define vStringPutWithLimit(s, c, l) \
+	(sizeof(c) == sizeof(char) \
+	 ? vStringPutWithLimitImpl((s), (unsigned char) (c), (l)) \
+	 : vStringPutWithLimitImpl((s), (c), (l)))
+
 #endif  /* CTAGS_MAIN_VSTRING_H */