coredump: accept hostname on command line (#8033)

This commint adds a new command line parameter to sytemd-coredump. The
parameter should be mappend to core_pattern's placeholder %h - hostname.

The field _HOSTNAME holds the name from the kernel's namespaces which might be
different then the one comming from process' namespaces.

It is true that the real hostname is usually available in the field
COREDUMP_ENVIRON (environment variables) but I believe it is more reliable to
use the value passed by kernel.

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The length of iovec is no longer static and hence I corrected the declarations
of the functions set_iovec_field and set_iovec_field_free.

Thank you @yuwata and @poettering!