syscalls: mounts: Fix a matching bug.

On some systems, the columns in /proc/self/mountinfo look like this:

23 28 0:21 / /proc rw,nosuid,nodev,noexec,relatime shared:11 - proc proc rw

Before this change, the mounts procedure was written with the assumption that
the type and source could always be found in columns 8 and 9, respectively.
However, the proc(5) man page explains that there can be zero or more optional
fields starting at column 7 (e.g., "shared:11" above), so this assumption is
false in some situations.

* guix/build/syscalls.scm (mounts): Update the match pattern to use ellipsis
to match zero or more optional fields followed by a single hyphen.  Remove the
trailing ellipsis, since multiple ellipses are not allowed in the same level.
The proc(5) man page indicates that there are no additional columns, so it is
probably OK to match an exact number of columns at the end like this.

guix/build/syscalls.scm

@@ -636,8 +636,9 @@ current process."
           (if (eof-object? line)
               (reverse result)
               (match (string-tokenize line)
+                ;; See the proc(5) man page for a description of the columns.
                 ((id parent-id major:minor root mount-point
-                     options _ type source _ ...)
+                     options _ ... "-" type source _)
                  (let ((devno (string->device-number major:minor)))
                    (loop (cons (%mount (octal-decode source)
                                        (octal-decode mount-point)