update COPYRIGHT file with additional contributor information
update INSTALL file with new information and better advice

the text covering an ill-advised procedure for 'bootstrapping' a new
musl-based system in-place is removed. new information on targets and
compilers is added. formatting improved. the remaining text is
adjusted to cover both usage with musl-gcc on a non-musl-based system
and upgrading a musl-based system or toolchain.
update README in preparation for release

reduces the amount of news-like content on progress and development
direction and focuses on the present.
remove claim of XSI coverage from README

in addition to the dbm functions (which we don't intent to implement
anyway), fmtmsg is still missing too. rather than adding exceptions I
think it's best just to avoid making the claim.
release 1.0.0