Fix building bsd/.. after using `make obj`

[ngie-eign]

Refer to source files using .CURDIR instead of relying on the build to be done with NO_OBJ, etc.

Adjust .gitignore to ignore files left behind as part of FreeBSD's bsd.prog.mk and to also prevent git from overzealously ignoring bsd/dma{,-mbox-create}/....