ldattach: add --iflag command line option

ldattach: add --iflag command line option

Add a command line option '-i' / '--iflag' for setting or clearing
input flags on the serial device before attaching the line discipline.

[kzak@redhat.com: - use generic functions for work with iflags table
                  - add list of iflags to usage/help output
                  - move iflags parsing to separate function]

Impact: added functionality
Signed-off-by: Tilman Schmidt <tilman@imap.cc>
Signed-off-by: Karel Zak <kzak@redhat.com>