CLEANUP: tools: Make errptr const in `parse_line()`

This change is for consistency with `cfg_eval_condition()`.
diff --git a/include/haproxy/tools.h b/include/haproxy/tools.h
index 2e3adf2..e376a71 100644
--- a/include/haproxy/tools.h
+++ b/include/haproxy/tools.h
@@ -891,7 +891,7 @@
  * some expansion is made.
  */
 char *env_expand(char *in);
-uint32_t parse_line(char *in, char *out, size_t *outlen, char **args, int *nbargs, uint32_t opts, char **errptr);
+uint32_t parse_line(char *in, char *out, size_t *outlen, char **args, int *nbargs, uint32_t opts, const char **errptr);
 size_t sanitize_for_printing(char *line, size_t pos, size_t width);
 void update_word_fingerprint(uint8_t *fp, const char *word);
 void make_word_fingerprint(uint8_t *fp, const char *word);
diff --git a/src/cfgparse.c b/src/cfgparse.c
index fa2072a..0f8ad57 100644
--- a/src/cfgparse.c
+++ b/src/cfgparse.c
@@ -1959,7 +1959,7 @@
 
 		while (1) {
 			uint32_t err;
-			char *errptr;
+			const char *errptr;
 
 			arg = sizeof(args) / sizeof(*args);
 			outlen = outlinesize;
diff --git a/src/tools.c b/src/tools.c
index 718bb49..bd6bf4e 100644
--- a/src/tools.c
+++ b/src/tools.c
@@ -5183,7 +5183,7 @@
  * the string. One solution for the caller may consist in replacing all args
  * delimiters with spaces in this case.
  */
-uint32_t parse_line(char *in, char *out, size_t *outlen, char **args, int *nbargs, uint32_t opts, char **errptr)
+uint32_t parse_line(char *in, char *out, size_t *outlen, char **args, int *nbargs, uint32_t opts, const char **errptr)
 {
 	char *quote = NULL;
 	char *brace = NULL;