AP_Param: make fgets buflen semantics match C's fgets

buflen now includes the space required for the null terminator, so up
to buflen-1 characters are returned.  Previously fgets could write
buf[buflen], one byte beyond the caller-nominated length.  All
in-tree callers passed sizeof(buf)-1 to compensate, but
posix_compat's apfs_fgets forwards its C-style size argument
directly, so a caller supplying a size-byte buffer could have that
buffer overrun by one byte on an over-long line.

Update callers to pass the full buffer size; usable capacity is
unchanged.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

libraries/AP_Param/AP_Param.cpp

@@ -2327,7 +2327,7 @@ bool AP_Param::count_defaults_in_file(const char *filename, uint16_t &num_defaul
     /*
       work out how many parameter default structures to allocate
      */
-    while (AP::FS().fgets(line, sizeof(line)-1, file_apfs)) {
+    while (AP::FS().fgets(line, sizeof(line), file_apfs)) {
         char *pname;
         float value;
         bool read_only;
@@ -2356,7 +2356,7 @@ bool AP_Param::read_param_defaults_file(const char *filename, bool last_pass, ui
 
     bool done_all = true;
     char line[100];
-    while (AP::FS().fgets(line, sizeof(line)-1, file_apfs)) {
+    while (AP::FS().fgets(line, sizeof(line), file_apfs)) {
         char *pname;
         float value;
         bool read_only;