awk: Use random(3) instead of rand(3)

While none of them is considered even near to cryptographic
level, random(3) is a better random generator than rand(3).

Use random(3) for awk as is done in other systems.

Thanks to Chenguang Li for discussing this in the lists
and submitting the patch upstream.

PR:		193147
MFC after:	5 weeks


git-svn-id: svn+ssh://svn.freebsd.org/base/head@271879 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f