net/vnstat: Clean up Makefile

- Use INSTALL_TARGET=install-exec to install the binary executables
- Use GNU_CONFIGURE_MANPREFIX and INSTALL_TARGET=install-man to install the manpages
- Use USES=localbase and CONFIGURE_ARGS=--disable-extra-paths to support non-default LOCALBASE
- Add CONFIGURE_ARGS=--disable-image-output when GUI option is disabled
- Bump PORTREVISION for package change

PR:		273231