Permit BAM headers between 2GB and 4GB in size once more.

This isn't permitted by the BAM specification, but was accepted by
earlier htslib release.  62f9909 added code to check the maximum
length.  This now has a warning at 2GB and the hard-failure at 4GB.

Fixes samtools#1420.  Fixes samtools/samtools#1613

sam.c

@@ -339,17 +339,23 @@ int bam_hdr_write(BGZF *fp, const sam_hdr_t *h)
 
     if (h->hrecs) {
         if (sam_hrecs_rebuild_text(h->hrecs, &hdr_ks) != 0) return -1;
-        if (hdr_ks.l > INT32_MAX) {
+        if (hdr_ks.l > UINT32_MAX) {
             hts_log_error("Header too long for BAM format");
             free(hdr_ks.s);
             return -1;
+        } else if (hdr_ks.l > INT32_MAX) {
+            hts_log_warning("Header too long for BAM specification (>2GB)");
+            hts_log_warning("Output file may not be portable");
         }
         text = hdr_ks.s;
         l_text = hdr_ks.l;
     } else {
-        if (h->l_text > INT32_MAX) {
+        if (h->l_text > UINT32_MAX) {
             hts_log_error("Header too long for BAM format");
             return -1;
+        } else if (h->l_text > INT32_MAX) {
+            hts_log_warning("Header too long for BAM specification (>2GB)");
+            hts_log_warning("Output file may not be portable");
         }
         text = h->text;
         l_text = h->l_text;