* geth,utils: implement configurable developer account options

Prior to this change --dev (developer) mode
generated one account with an empty password,
irrespective of existing --password and --miner.etherbase
options.

This change makes --dev mode compatible with these
existing flags.

--dev mode may now be used in conjunction with
--password and --miner.etherbase flags to configure
the developer faucet using an existing keystore or
in creating a new account.

Signed-off-by: meows <b5c6@protonmail.com>

* main: remove key/pass flags from usage developer section

These flags are included already in other sections,
and it is not desired to duplicate them.

They were originally included in this section
along with added support for these flags in the
developer mode.

Signed-off-by: meows <b5c6@protonmail.com>