getdigits: introduce strict, def parameters

Problem:
During a refactor long ago, we changed the `getdigits_*` familiy of
functions to abort on overflow.  But this is often wrong, because many
of these codepaths are handling user input.

Solution:
Decide at each call-site whether to use "strict" mode.

fix #5555

src/nvim/indent_c.c

@@ -1678,7 +1678,7 @@ void parse_cino(buf_T *buf)
     if (*p == '-')
       ++p;
     char_u *digits_start = p;             /* remember where the digits start */
-    int n = getdigits_int(&p);
+    int n = getdigits_int(&p, true, 0);
     divider = 0;
     if (*p == '.') {        /* ".5s" means a fraction */
       fraction = atoi((char *)++p);
@@ -1915,7 +1915,7 @@ int get_c_indent(void)
         else if (*p == COM_LEFT || *p == COM_RIGHT)
           align = *p++;
         else if (ascii_isdigit(*p) || *p == '-') {
-          off = getdigits_int(&p);
+          off = getdigits_int(&p, true, 0);
         }
         else
           ++p;