introduce /usr/{,local/}libexec directories

introduce /usr/{,local/}libexec directories

https://refspecs.linuxfoundation.org/FHS_3.0/fhs/ch04s07.html

Note, that /usr/local/libexec is not mentioned explicitly in
https://refspecs.linuxfoundation.org/FHS_3.0/fhs-3.0.html#requirements10,
but rationale behind both libexec and local hierarchy allow us to treat
this as lib<qual>.

Since /usr/local/libexec should exist for GNU applications according to
https://www.gnu.org/prep/standards/html_node/Directory-Variables.html,
we must provide /usr/libexec directory as well to met FHS requirements.