use grep -E instead of egrep

[lufia]

I noticed egrep always output warning:

egrep: warning: egrep is obsolescent; using grep -E

I heared this warning may be introduced in GNU Grep v3.8.

[dancrossnyc]

Not all systems use GNU grep, however, and the world as a whole does not seem to be in a rush to get rid of egrep. I find the warnings annoying; some uses of egrep can probably just be replaced with grep (without the -E) which would, at least, cut down on the noise.

On the other hand, grep -E does seem to be in POSIX (https://pubs.opengroup.org/onlinepubs/9699919799/utilities/grep.html) so it may be sufficiently portable, and POSIX does make a note about the intention to combine grep, egrep and fgrep into a single command. However, I am not sure that everything that plan9port supports has gotten the memo yet.

[lufia]

According to https://9fans.github.io/plan9port/

Linux (x86, x86-64, PowerPC, and ARM), FreeBSD (x86, x86-64), Mac OS X (x86, x86-64, and Power PC), NetBSD (x86 and PowerPC), OpenBSD (x86 and PowerPC), SunOS (x86-64 and Sparc), Dragonfly BSD (x86-64).

I checked grep supports -E option nowadays on several systems:

• Linux: https://www.gnu.org/software/grep/manual/grep.html
• FreeBSD: https://man.freebsd.org/cgi/man.cgi?grep
• macOS(I couldn't find official man page on the web): https://www.unix.com/man-page/osx/1/grep/
• NetBSD: https://man.netbsd.org/grep.1
• OpenBSD: https://man.openbsd.org/grep.1
• Solaris: https://docs.oracle.com/cd/E23823_01/html/816-5165/grep-1.html
• Dragonfly BSD: https://leaf.dragonflybsd.org/cgi/web-man?command=grep&section=1

The -E option means to allow extended regexp in all implementations of grep(1).

[macie]

This PR fixes #587