emerge: check for writable PKGDIR (490732)

If there are packages to be merged and "buildpkg" or "buildsyspkg" is
enabled, then bail out early if PKGDIR is not writable (in order to
avoid a fatal EROFS error which would otherwise occur later on).
Behavior remains unchanged for --pretend, --fetchonly and
--fetch-all-uri. For --ask, it will bail out just after the last
relevant --ask prompt.

X-Gentoo-Bug: 490732
X-Gentoo-Bug-URL: https://bugs.gentoo.org/show_bug.cgi?id=490732