elf: Do not parse ill-formatted strings

Instead of ignoring ill-formatted tunable strings, first, check all the
tunable definitions are correct and then set each tunable value. It
means that partially invalid strings, like "key1=value1:key2=key2=value'
or 'key1=value':key2=value2=value2' do not enable 'key1=value1'. It
avoids possible user-defined errors in tunable definitions.

Checked on x86_64-linux-gnu.
Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>