for every new added htlc, check that adding it won't go over our 'dust
budget' (which assumes a slightly higher than current feerate, as this
prevents sudden feerate changes from overshooting our dust budget)
note that if the feerate changes surpass the limits we've set, we
immediately fail the channel.
Note that check-whitespace and check-bolt already do this, so we
can eliminate redundant lines in common/Makefile and bitcoin/Makefile.
We also include the plugin headers in ALL_C_HEADERS so they get
checked.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>