[PATCH] umount: always use umount2 syscall with specified flags

[Denys Vlasenko]

I committed several changes which adopt kernel-style BUILD_BUG_ON:

#define BUILD_BUG_ON(condition) ((void)sizeof(char[1 - 2*!!(condition)]))

Need some analogue for declaration-level check.
BUILD_BUG_ON would not work here:

typedef struct svstatus_t {
        uint64_t time_be64 PACKED;
        uint32_t time_nsec_be32 PACKED;
        uint32_t pid_le32 PACKED;
        uint8_t  paused;
        uint8_t  want; /* 'u' or 'd' */
        uint8_t  got_term;
        uint8_t  run_or_finish;
} svstatus_t;
struct ERR_svstatus_must_be_20_bytes {
        char ERR_svstatus_must_be_20_bytes[sizeof(svstatus_t) == 20 ? 1 : -1];
};