preparing for the syscall package freeze.

««« original CL description
syscall: consolidate, simplify socket options for Unix-like systems

Also exposes common socket option functions on Solaris.

Update #7174
Update #7175

LGTM=aram
R=golang-codereviews, aram
CC=golang-codereviews
https://golang.org/cl/107280044
»»»

LGTM=r
R=r
CC=golang-codereviews
https://golang.org/cl/121880043