[misc] Consolidate control file reader (e.g. for pairing credentials) into misc.c

- also remove requirement to enter device name in .remote file

src/misc.h

@@ -80,6 +80,9 @@ keyval_sort(struct keyval *kv);
 char *
 m_realpath(const char *pathname);
 
+char **
+m_readfile(const char *path, int num_lines);
+
 char *
 unicode_fixup_string(char *str, const char *fromcode);