fix the completion problem of `man` under OS X

The problem is `apropos` under OS X behaves differently from Linux.

1. The format: for command `apropos sudo`, the output under Linux:
```
sudo (8)             - execute a command as another user
```
and the output under OS X:
```
sudo(8)                  - execute a command as another user
```

one-space difference.

2. For some items sharing a same manpage, e.g. for `apropos printf`,

Linux:
```
printf (3)           - formatted output conversion
snprintf (3)         - formatted output conversion
sprintf (3)          - formatted output conversion
vprintf (3)          - formatted output conversion
vsnprintf (3)        - formatted output conversion
vsprintf (3)         - formatted output conversion
```

OS X:
```
printf(3), fprintf(3), sprintf(3), snprintf(3), asprintf(3), dprintf(3), vprintf(3), vfprintf(3), vsprintf(3), vsnprintf(3), vasprintf(3), vdprintf(3) - formatted output conversion
```

OS X puts them in one line.

So, we need special handling here, otherwise, completion with prefix
`man 3 printf` will give the result of "man 3 printf\(3\),"