Audit sort(1) and mark it as finished

1) Remove the function prototypes. No need for them, as the
   functions are ordered.
2) Add fieldseplen, so the length of the field-separator is not
   calculated nearly each time skipcolumn() is called.
3) rename next_col to skip_to_next_col so the purpose is clear,
   also reorder the conditional accordingly.
4) Put parentheses around certain ternary expressions.
5) BUGFIX: Don't just exit() in check(), but make it return something,
   so we can cleanly fshut() everything.
6) OFF-POSIX: Posix for no apparent reason does not allow more than
   one file when the -c or -C flags are given.
   This can be problematic when you want to check multiple files.
   With the change 5), rewriting check() to return a value, I went
   off-posix after discussing this with Dimitris to just allow
   arbitrary numbers of files. Obviously, this does not break scripts
   and is convenient for everybody who wants to quickly check a big
   amount of files.
   As soon as 1 file is "unsorted", the return value is 1, as expected.
   For convenience reasons, check()'s warning now includes the filename.
7) BUGFIX: Set ret to 2 instead of 1 when the fshut(fp, *argv) fails.
8) BUGFIX: Don't forget to fshut stderr at the end. This would improperly
   return 1 in the following case:
   $ sort -c unsorted_file 2> /dev/full
9) Other style changes, line length, empty line before return.

README

@@ -69,7 +69,7 @@ The following tools are implemented:
 =*|x sha256sum   .
 =*|x sha512sum   .
 =*|o sleep       .
-#    sort        (-d, -f, -i)
+#*|o sort        (-d, -f, -i)
 =*|o split       .
 =*|x sponge      .
 #*|o strings     .