Constify based on Flexelint analysis

src/tus_blob.c

@@ -258,7 +258,7 @@ tus_chksum_hdr(VRT_CTX, const char *hdr)
 }
 
 void
-tus_chksum_update(VRT_CTX, struct tus_chksum *c,
+tus_chksum_update(VRT_CTX, const struct tus_chksum *c,
     const void *ptr, size_t l)
 {
 	struct vrt_blob b;

src/tus_blob.h

@@ -31,7 +31,7 @@ int tus_chksum_fini(VRT_CTX);
 struct vmod_blobdigest_digest *tus_hash(const char *, size_t);
 struct tus_chksum *tus_chksum_new(VRT_CTX, struct vmod_blobdigest_digest *);
 struct tus_chksum *tus_chksum_hdr(VRT_CTX, const char *);
-void tus_chksum_update(VRT_CTX, struct tus_chksum *c,
+void tus_chksum_update(VRT_CTX, const struct tus_chksum *c,
     const void *ptr, size_t l);
 VCL_BLOB tus_chksum_final(VRT_CTX, struct tus_chksum *c);
 VCL_BOOL tus_chksum_equal(VRT_CTX, struct tus_chksum *c);

src/vmod_tus.c

@@ -164,7 +164,8 @@ tus_schemeauth_valid(const char *schemeauth, const char **p)
 }
 
 static struct VPFX(tus_server) *
-tus_server_new(VRT_CTX, const char *vcl_name, struct VARGS(server__init) *args)
+tus_server_new(VRT_CTX, const char *vcl_name,
+    const struct VARGS(server__init) *args)
 {
 	struct VPFX(tus_server) *tussrv;
 	const char *basedir, *p;
@@ -213,7 +214,7 @@ tus_server_new(VRT_CTX, const char *vcl_name, struct VARGS(server__init) *args)
 
 static struct VPFX(tus_server) *
 tus_server_ref(VRT_CTX, struct VPFX(tus_server) *tussrv,
-    struct VARGS(server__init) *args)
+    const struct VARGS(server__init) *args)
 {
 	const char *basedir = tussrv->basedir;