Rollup merge of #41054 - anatol:master, r=alexcrichton
Rollup merge of #41054 - anatol:master, r=alexcrichton

Replace magic number with readable sig constant

SIG_ERR is defined as 'pub const SIG_ERR: sighandler_t = !0 as sighandler_t;'

GBGamer/rust,GBGamer/rust,graydon/rust,GBGamer/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,GBGamer/rust,graydon/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,graydon/rust,graydon/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,aidancully/rust,aidancully/rust