support 'nethack --nethackrc=~/File'

Substitute $HOME/File if command line specifies --nethackrc=~/File
to avoid "Access to ~/File denied (2)".

Only implemented for opening run-time config file on Unix.  Works for
NETHACKOPTIONS=@~/File too; the normally optional at-sign is required
since the tilde won't match a slash to distinguish file versus options.

Only supports "~/" file path prefix, not "~user/".

doc/fixes3-7-0.txt

@@ -2479,6 +2479,8 @@ Unix: add ../include/nhlua.h to the alloc.o dependencies in Makefile.utl to
 Unix: implement SELF_RECOVER compile-time option, on by default on linux
 Unix: allow build to succeed with musl (instead of glibc), by specifying
 	musl=1 on the make command line
+Unix: support tilde expansion for home directory path, "~/relative-path", in
+	command line --nethackrc=path and environment NETHACKOPTIONS=@path
 user_sounds: move the message hook from inside individual window display ports
 	to the core where it allows MSGTYP_NOSHOW msgtyp's to still trigger
 	sounds to correct a reported github issue; also fixes a past reported

src/cfgfiles.c

@@ -242,6 +242,13 @@ fopen_config_file(const char *filename, int src)
     if (filename && *filename) {
         set_configfile_name(filename);
 #ifdef UNIX
+        if (!strncmp(configfile, "~/", 2) && (envp = nh_getenv("HOME")) != 0) {
+            /* support for command line '--nethackrc=~/path' (or for
+               NETHACKOPTIONS='@~/path'; we don't support ~user/path) */
+            Snprintf(tmp_config, sizeof tmp_config, "%s/%s",
+                     envp, configfile + 2); /* insert $HOME/ and remove ~/ */
+            set_configfile_name(tmp_config);
+        }
         if (access(configfile, 4) == -1) { /* 4 is R_OK on newer systems */
             /* nasty sneaky attempt to read file through
              * NetHack's setuid permissions -- this is the only