Sgrep (sorted grep) searches sorted input files for lines that match a
search key and outputs the matching lines. When searching large files
sgrep is much faster than traditional Unix grep, but with significant
restrictions.

 * All input files must be sorted regular files.
 * The sort key must start at the beginning of the line.
 * The search key matches only at the beginning of the line.
 * No regular expression support.
