claws-mail: fix attachment MIME type detection

Adds support for shared-mime-info to Claws, to fix attachments in
outgoing messages always having MIME type application/octet-stream
because Claws doesn't know where to look, instead complaining:

/nix/store/...-claws-mail-3.11.1/etc/mime.types: fopen: No such file or directory

Moreover, Claws relies on incoming MIME types for knowing when e.g. to
display an attached image, so sending application/octet-stream
unnecessarily is bad.

Tested against release-15.09.

pkgs/applications/networking/mailreaders/claws-mail/mime.patch

@@ -0,0 +1,14 @@
+--- a/src/procmime.c	2015-09-18 04:03:11.767654094 -0700
++++ b/src/procmime.c	2015-09-18 04:08:38.834503034 -0700
+@@ -1196,11 +1196,7 @@
+ 	if (mime_type_list) 
+ 		return mime_type_list;
+ 	
+-#if defined(__NetBSD__) || defined(__OpenBSD__) || defined(__FreeBSD__)
+ 	if ((fp = procmime_fopen(DATAROOTDIR "/mime/globs", "rb")) == NULL) 
+-#else
+-	if ((fp = procmime_fopen("/usr/share/mime/globs", "rb")) == NULL) 
+-#endif
+ 	{
+ 		fp_is_glob_file = FALSE;
+ 		if ((fp = procmime_fopen("/etc/mime.types", "rb")) == NULL) {